In a world where five artificial islands exists, Mamoru is a "mermaid" transferred to one of those islands. When Mamoru is attacked, a newly transferred girl named Mirei saves her. The enemy doesn't stop, however, and the two are soon cornered. Just when they think all hope is lost, Mirei kisses Mamori, and Mamoru turns into a sword. Mirei then wields the sword and launches a counterattack against their enemies.